This invention relates to a variable reluctance rotation sensor which generates a high output signal at very low rotational speeds. Such a sensor would find particular utility in an vehicle anti-lock braking or traction control, in which low rotational speed sensing is required.
A variable reluctance rotation sensor of the prior art may typically comprise a rotatable toothed magnetic wheel and a stationary sensor unit having a permanent magnet and apparatus establishing a main flux loop including the permanent magnet and toothed wheel and defining an air gap between a member of the sensor unit and the closest tooth or teeth of the wheel. The member defining the air gap may be a magnetic flux member or the permanent magnet itself. The air gap thus varies with rotation of the toothed wheel between a small gap when the member is aligned with a tooth and a larger gap when it is not. The main flux loop is linked to all the turns of an electrical coil surrounding the magnetic flux member; and an electric voltage is generated in the coil proportional to the rate of change of the flux linked thereto. As the toothed wheel rotates, the total flux in the main flux loop linked to the coil changes with the changing air gap, as teeth and inter-teeth slots alternately pass the stationary sensor unit; and an output electrical signal is generated in a sinusoidal pattern, with a maximum each time the member is aligned with a tooth and a minimum each time it is aligned with a slot.
The amplitude of the signal in a variable reluctance rotation sensor of the prior art varies with such well known design factors as the energy product of the permanent magnet and the minimum air gap size; and good design in the prior art has tended to provide the greatest magnetic energy from a magnet of a given size, the smallest consistently producible minimum air gap and the greatest possible concentration of main flux through the tooth adjacent the magnet or flux member. Although leakage flux, which does not follow the main flux loop through both magnets and the full coil, is inescapable, the design tendency has been to minimize it for the greatest possible percentage of total magnetic flux in the main flux loop linked to the entire coil. The recent use of new high energy product magnetic materials has helped provide a high level of magnetic flux from a small magnet in a limited space. However, it is difficult to hold a tight minimum air gap consistently in mass production at reasonable cost. Also, the amplitude of the sensor signal decreases with rotational speed, since the rate of change of flux due to air gap variation decreases as the tooth moves more slowly past the stationary member. Therefore, inexpensive variable reluctance rotation sensors generating a high output at very low rotational speeds are not readily available.
In a vehicle anti-lock brake system, the sinusoidal output of a variable reluctance rotation sensor is filtered with a noise suppressing dead band. A rotation is counted only if the sinusoidal peak of the signal exceeds the deadband, with any signal event within the deadband not accompanied by travel out of the deadband being considered noise and ignored. The greater the peak-to-peak signal level, the greater the deadband can be established and the greater is noise immunity. However, as rotational speed falls close to zero RPM, the output of most available rotational speed sensors falls to very low levels which can limit the usable deadband and thus reduce noise immunity. This makes it more difficult to use such sensors in anti-lock braking systems.